That is because they accepted faith with their tongues then disbelieved with their hearts, therefore their hearts were sealed – so now they do not understand anything.
इस सबब से कि (ज़ाहिर में) ईमान लाए फिर काफ़िर हो गए, तो उनके दिलों पर (गोया) मोहर लगा दी गयी है तो अब ये समझते ही नहीं